From the Writings of Bahá'u'lláh:

We testify that He is One in His Essence, One in His Attributes. He hath none to equal Him in the whole universe, nor any partner in all creation.

(Bahá'u'lláh: Epistle to the Son of the Wolf, p. 98)

He, in truth, hath, throughout eternity, been one in His Essence, one in His attributes, one in His works. Any and every comparison is applicable only to His creatures, and all conceptions of association are conceptions that belong solely to those that serve Him. Immeasurably exalted is His Essence above the descriptions of His creatures. He, alone, occupieth the Seat of transcendent majesty, of supreme and inaccessible glory.

(Bahá'u'lláh: Gleanings, p. 193)

Regard thou the one true God as One Who is apart from, and immeasurably exalted above, all created things. The whole universe reflecteth His glory, while He is Himself independent of, and transcendeth His creatures. This is the true meaning of Divine unity. He Who is the Eternal Truth is the one Power Who exerciseth undisputed sovereignty over the world of being, Whose image is reflected in the mirror of the entire creation. All existence is dependent upon Him, and from Him is derived the source of the sustenance of all things. This is what is meant by Divine unity; this is its fundamental principle.

Some, deluded by their idle fancies, have conceived all created things as associates and partners of God, and imagined themselves to be the exponents of His unity. By Him Who is the one true God! Such men have been, and will continue to remain, the victims of blind imitation, and are to be numbered with them that have restricted and limited the conception of God.

He is a true believer in Divine unity who, far from confusing duality with oneness, refuseth to allow any notion of multiplicity to becloud his conception of the singleness of God, who will regard the Divine Being as One Who, by His very nature, transcendeth the limitations of numbers.

The essence of belief in Divine unity consisteth in regarding Him Who is the Manifestation of God and Him Who is the invisible, the inaccessible, the unknowable Essence as one and the same. By this is meant that whatever pertaineth to the former, all His acts and doings, whatever He ordaineth or forbiddeth, should be considered, in all their aspects, and under all circumstances, and without any reservation, as identical with the Will of God Himself. This is the loftiest station to which a true believer in the unity of God can ever hope to attain. Blessed is the man that reacheth this station, and is of them that are steadfast in their belief.

(Bahá'u'lláh: Gleanings, pp. 166-167)

Whatsoever in the contingent world can either be expressed or apprehended, can never transgress the limits which, by its inherent nature, have been imposed upon it. God, alone, transcendeth such limitations. He, verily, is from everlasting. No peer or partner has been, or can ever be, joined with Him. No name can be compared with His Name. No pen can portray His nature, neither can any tongue depict His glory. He will, for ever, remain immeasurably exalted above any one except Himself.

(Bahá'u'lláh: Gleanings, p. 151)

And now concerning thy reference to the existence of two Gods. Beware, beware, lest thou be led to join partners with the Lord, thy God. He is, and hath from everlasting been, one and alone, without peer or equal, eternal in the past, eternal in the future, detached from all things, ever-abiding, unchangeable, and self-subsisting. He hath assigned no associate unto Himself in His Kingdom, no counsellor to counsel Him, none to compare unto Him, none to rival His glory.

(Bahá'u'lláh: Gleanings, p. 192)

From the Writings of the Báb:
God testifieth that there is none other God but Him. His are the kingdoms in the heavens and on the earth and all that is between them. He is exalted above the comprehension of all things, and is inscrutable to the mind of every created being; none shall be able to fathom the oneness of His Being or to unravel the nature of His Existence. No peer or likeness, no similitude or equal can ever be joined with Him.

(The Báb: Selections from the Báb, p. 154)

From the Utterances of 'Abdu'l-Bahá:
The great and fundamental teachings of Bahá'u'lláh are the oneness of God and unity of mankind.

('Abdu'l-Bahá: Promulgation of Universal Peace, p. 156)

Bahá'u'lláh appeared upon the horizon of the East, proclaiming the oneness of God and the unity of the world of humanity. He promulgated the teaching that all mankind are the servants of one God; that all have come into being through the bestowal of the one Creator; that God is kind to all, nurtures, rears and protects all, provides for all and extends His love and mercy to all races and people.

('Abdu'l-Bahá: Promulgation of Universal Peace, p. 174)

There is also the divine unity or entity, which is sanctified above all concept of humanity. It cannot be comprehended nor conceived because it is infinite reality and cannot become finite. Human minds are incapable of surrounding that reality because all thoughts and conceptions of it are finite, intellectual creations and not the reality of Divine Being which alone knows itself. For example, if we form a conception of Divinity as a living, almighty, self-subsisting, eternal Being, this is only a concept apprehended by a human intellectual reality. It would not be the outward, visible reality, which is beyond the power of human mind to conceive or encompass. We ourselves have an external, visible entity; but even our concept of it is the product of our own brain and limited comprehension. The reality of Divinity is sanctified above this degree of knowing and realization. It has ever been hidden and secluded in its own holiness and sanctity above our comprehending. Although it transcends our realization, its lights, bestowals, traces and virtues have become manifest in the realities of the Prophets, even as the sun becomes resplendent in various mirrors. These holy realities are as reflectors, and the reality of Divinity is as the sun, which, although it is reflected from the mirrors, and its virtues and perfections become resplendent therein, does not stoop from its own station of majesty and glory and seek abode in the mirrors; it remains in its heaven of sanctity. At most it is this: that its lights become manifest and evident in its mirrors or manifestations. Therefore, its bounty proceeding from them is one bounty, but the recipients of that bounty are many. This is the unity of God; this is oneness - unity of Divinity, holy above ascent or descent, embodiment, comprehension or idealization - divine unity.

('Abdu'l-Bahá: Promulgation of Universal Peace, pp. 192-193)